Temperature regulating system



Patented Feb. 22, 1949 TEMPERATURE REGULATING SYSTEM Clarence L. Mershon, Lima, Ohio, assignor to Westinghouse Electric Corporation, East Pittsburg-h, Pa., a corporation of Pennsylvania Application May 28, 1947, Serial No. 750,904

Claims. 1

This invention relates to regulating systems, and in particular to a system for preventing the frosting of windows.

In the aircraft industry considerable efforts have been made to prevent the frosting of windows. The conditions encountered in service by such windows are quite variable, rendering the problem difiicult to solve. Recently a window glass manufacturer has produced a windowpane formed of laminated sections of plastic and glass in which the surface of one or more panes of glass is coated with a transparent conducting material, which when connected in an electrical circuit will act as a resistor and dissipate heat within the window. In

practice, the variable weather conditions encountered have rendered it difficult to maintain a constant temperature in such windows with the result that under certain weather temperature conditions the window is satisfactorily defrosted, while under other conditions, the window becomes so frosted as to render it nontransparent.

An object of this invention is to provide for regulating the temperature of a window exposed to various weather conditions to prevent frosting thereof.

Another object of this invention is to provide for heating a window at a substantially constant temperature.

A more specific object of this invention is to provide, in a system for heating a window, for.

controlling the input to the window heater in accordance with changes in temperature of the Window from a predetermined temperature which is to be maintained.

A further specific object of this invention is to provide a regulating system having a res'is'tance bridge circuit sensitive to changes in temperature for detecting variations in the temperature of a window for effecting a corrective action to change the power input to a resistance heating element associated with the window.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawin s, in which:

fi ures 1, 2, '3 and 4 are diagrammatic views ill 2 of apparatus and circuits involving the teachings of this invention.

Referring to the drawings, and in particular to Fig. 1 thereof, this invention is illustrated by reference to a system for controlling the temperature of a windowpane ID. The windowpane ill in this instance is formed of a layer of plastic l2 sandwiched between two layers of glass l4, one layer of glass having a coating l6 formed of a conductive material which will function as a resistor element therebetween. Such a windowpane I0 is more fully described in Technical Glass Bulletin of the Pittsburgh Plate Glass Company of Pittsburgh, Pennsylvania. The composition of the layers [2, l4 and 16 may be of many different mate- ,rials so long as the combination acts as a transparent windowpane and the conductive layer l6 functions as a resistor element or has resistor elements embedded therein as described hereinafter.

In order to supply current to the layer l6 of conductive material to effectively heat the windowpane ID, a generator I8 is provided, the generator being disposed to be driven by a suitable prime mover, not shown, and being formed of armature windings 20 and field windings 22. The armature windings 241 are connected by conductors 24 and 26 to the primary windings of a transformer 28, the secondary windings of which are connected to terminals formed on the intermediate layer l6 of conductive mate rial. As illustrated the field windings 22 are connected through a pile resistor 30 and by conductors 32 and 34 to a suitable source of direct current supply represented by the positive andnegative symbols.

In this instance the pile resistor 30 is of the usual construction having a pile 36 formed of a plurality of discs of high resistance material such as carbon or the like and being disposed to be compressed by means of the crank lever 38 which is normally biased to apply pressure thereto by a spring 40 and disposed to be moved to release the pressure by an electromagnet 42. The electromagnet t2 is provided with a Winding 44 which is disposed to be connected through a slide wire full-wave rectifier 48, the. input terminals of 3 which are connected across the conductors 24 and 26, whereby the winding 44 is normally supplied in accordance with the output of the generator i8.

In order to control the energization of the winding 46, the sliding contact member 53 of the slide wire rheostat 46 is disposed to be actuated by a reversible motor 52 which is connected to be operated in accordance with the energization. of a polarized relay 55. As illustrated the reversible motor 52 comprises the armature windings 56 and opposed field windings 58 and 60; one terminal of the armature windings being connected to conductor 32 and the other terminal being selectively connected through field winding 58, contact member 62 of a limit switch 63, conductor 64, the armature contact member'fifi of the polarized relay 5:3

and conductors 68 and lil to conductor 34, or through field winding 60, contact member E2 of a limit switch 13, conductor 14, the armature contact member 66 of the polarized relay 54 and conductors 68 and "ill to the conductor 35. The

contact members 62 and 12 of the limit switches 63 and 13, respectively, are disposed to be actuated to open circuitl position depending upon the direction of and the extent of movement of the sliding contact'membertil of the slide wire rheostat 46, thereby efiectively limiting the operation. of the slide wire rheostat 46. V

Asfillustrated, the polarized relay 54 is disposed to be directly energized in accordance with the unbalanced condition of a resistance bridge formed of resistors 16, 18,80 and 82. As illustrated, the resistors 16 and it comprise one leg of the resistor bridge, resistors 80 and 82 forming the other leg, the two legs being connected at their intermediatepoints 84 and 86 by conductors 'HJ and'88 to conductors 36 and 32, respectively. As illustrated, theenergizing winding of the polarized relay 54 isconnected to intermediate taps 9B and 92 in the legs of the resistance bridge.

In this embodiment the resistor 82 is formed of any material havin a high temperature coeificientof resistance, suchas nickel wire, the resistor being represented as being disposed closely adjacent to the windowpane l0 and, where pos sible, embedded in the plastic layer I2 of the win dow so that 'the resistance of the resistor 82 will vary directly as the temperature of the windov pane l0 varies from a predetermined temperature which is to be maintained. On the other hand, the resistors 16, {wand 18 are preferably formed of material having a low temperature coefficient of resistance, such as Advance, the resistor 18 also preferably being positioned close to the windowpane Ill so that the leads to-the resistors l8 and 82 will be approximately equal to eliminate errors in resistance within the bridge itself. The resistance bridge can be initially balanced by suitably adjusting the resistors and 80. The same effect can be obtained by maln'ng the resistance of leads 90 to 18 plus 18 to 84 equal to resistance of leads 84 to 82 plus 82 to 92.

In operation, assuming that the layer l6 of the windowpane Ill is connected to be supplied from the generator lB and that the resistance bridge has been properly balanced to maintain a predetermined temperature within the window, the system is as shown with the polarized relay 54 do energized and the armature contact member 66 thereof in a mid-position.

Assuming that the temperature of the windowpane l0 decreases from the predetermined tel Derature which it is desired to maintain, then the 4 resistance of resistor 82 decreases in accordance with the temperature decrease with the result that the polarity of the mid-terminal 92 of the resistance bridge becomes more positive with respect to the polarity of the mid-terminal 9i! and current fiows from the mid-terminal 92 through the winding of the polarized relay 55 to the mid terminal 96. When the winding of the polarized relay 54 is energized in this manner, the arma ture contact member 66 is moved to the left to close a circuit which extends from the conductor 36 through conductors ii! and 68, armature contact membertfi of the relay 54, conductor "i4, con tact member'l'Z of the limit switch 53, field winding 6% and armature windings 56 to the conductor 32 to so energize the reversible motor 52 as to drive the sliding contact member 53 in a counterclockwise direction to increase the resistance of the rheostat 46 in circuit with the energizing winding i l of electromagnet 52, whereby the spring member '55 functions to apply more pressure to the pile 355 of the pile rheostat 33 and thereby increase the fiow of current through the field winding 22 of the generator it. As the excitation of the generatorils is thus increased, the flow of current therefrom-to the conductive layer it of the windowpane is increased to thereby effect an increase in the temperature of the windowpane lilto return such temperature to :he predetermined:temperature which is to be maintained. i 7

On the other hand, if the temperature variation of the windowpane it is an increase in tempera-- ture, then the resistance of the resistor element 82 increases in accordance with the increase in temperature, whereby the polarity of the midterminal 94] becomes more positive with respect to the potential of the mid-terminal 92 and current fiows through the winding of the polarized relay 54 ina direction to actuate the armature contact member 66 of the relay 5@ to the right to close a circuit which extends from conductor 35 through conductors It and 68, armature contact member 56, conductor 64, contact member 62 of the limit switch 63, field winding 53 and armature windings 56 of the reversible motor 52 to conductor 32, whereby the motor 52 is energized to drive the sliding contact member 50 in a clockwise direction to decrease the resistance in circuit with the windin 44 of the electromagnet 42, whereby the electromagnet 42 operates in opposition to the spring member 4!! to release the pressure on the pile 36 of the rheostat and thereby increase the resistance thereofand decrease the flow of current through the field winding 22. As the excitation of the generator l8 is'thus decreased, the how of current from the generator 38 to the conducting layer It of'the windowpane ii? is decreased whereby the temperature developed within thewindowpane 10 is decreased towards the predetermined temperature which is to be maintained.

It is apparent that the operation of the system described is efiective for controlling or regulating the temperature of the windowpane it for as the resistance in the winding circuit of the electromagnet 42 is varied, the output of the generator it is correspondingly varied tending to return the ampere turns of the winding of the electromagnet 42 to its initial value to establish the balance between the magnetic pull of the electromagnet 42 and the pressure of the spring :38 and thereby regulate the pressure applied to the pile of the pile rheostat 30. The response of the system is very sensitive, being directly controlled by the temperature variations within the windowpane I0.

The system shown in Fig. 2-is a simplified circult-for accomplishing substantially the same result as is accomplished with the circuit of Fig. l. In this embodiment, the same numerals are utilizedtfor'indicating identical portions of the circuit as has been described heretofore with respect to Fig. 1. 'In this instance, however, a fixed resistor 94 is utilized instead of the slide wire rheostat 46 of Fig. 1, the connections of the fixed resistor 94 in circuit with the Winding 4'4 of the electromagnetfl being directly controlled by the energization of the relay 96 which is connected across the mi'd-termina1s'90 and 92 of the resistance bridge, a 1'ectifier-98-b'eing provided in circuit with the winding of the relay 9'5 to control the direction of flow of current therethrough. As illustrated, the fixedresistor 94 is normally shunted from the winding circuit by the armature contact member I of polarized relay 9B which for a balanced condition of the resistance bridge is in circuit closing position with -a fixed contact member I0 I.

In addition to the polarized relay 96, 'a second relay I02 is provided for controlling the energizing circuit of the fie1d winding 22 of the generator I8 under predetermined operating conditions. In this instance, the relay I02 is disposed with its winding connected through a rectifier I05 across the mid-terminals 90 and 92 of the resistan'ce bridge, the armature contact member I06 of the relay I02 being normally in circuit closing position with fixed contact member I08 to close an energizing circuit to the'winding Iii? of a relay II2, the contact member of which normally bridges contact members H4 and H6.

In operation, assuming that the armature contact member I00 is in engagement with contact member I08 to energize the winding III) of relay H2 so that the fie1d winding 22 of generator I8 is energized and that the bridge has been adjust'ed so that it is in balance at a predetermined temperature within the Windowpane I0, then assuming that the temperature of the Windowpane decreases so that the potential at mid'- terminal 92 is more positive with respect to the potential at mid-terminal 90, current flows through the rectifier 98 and the winding of the polarized relay 96 to the mid-terminal 00 to energize the relay 96 and actuate its armature contact member I00 to a circuit opening position away from the fixed contact member I 02. As the circuit is interrupted by reason of the movement of armature contact member I00, resistor 84 is connected in series with the winding 44 of the electromagnet 42, whereby the magnetic puil of electromagnet 42' is decreased and the spring member 40 functions to apply more pressure to the pile 36 of the pile rheostat 30 to effectively increase the current flow through the fie1d winding 22. As the excitation of the generator I0 is thus increased, the current fiow to the conducting coating I0 of the windowpane I0 is increased to effect an increase in the temperature within the Window.

On the other hand, if the temperature of the window pane I0 is increased above the mode" termined temperature which is to be maintained, then the potential-at mid-terminal 90 becomes more positive with respect to the potential of the mid-terminal 52 and current flows from the terminal 90 through the winding of the relay I02, rectifier I04 to the terminal-92 to so "energize the relay I 02 as to actuate 'the armature. contact member I06 to the right, away from the fixed contact member I08 to interrupt the energizing circuit to the winding H0 of relay II2, whereby relay H2 functions to permit its contact member to drop away from the contact members H5 and H0, thereby interrupting the circuit to the field winding 22 to effect the deenergization thereof. The interruption of the energizing circuit of field winding 22, of course, interrupts the how of current from the generator 58 to the conducting coating I0 of the windowpane I0, whereby the temperature of the window decreases towards the predetermined temperature which is to be maintained.

As in the embodiment of Fig. l, the circuit illustrated in Fig. 2 is likewise very sensitive to temperature changes and. is prompt and positive in its operation to efiect an increase or decrease in the temperature of the windowpane I0 to maintain a predetermined temperature which is required.

The embodiment of the invention illustrated in 3 is somewhat similar to that shown in Fig. 2, a more sensitive control of the energize.- tion of the winding 40 of relay 42, however, being obtained. In this instance, the connection of sections of the resistor s0 is controlled directly by polarized relays and IE2, the resistor being provided with a mid-terminal [I8 which is connected to the armature contact members M0 and I06 of the relays 06 and I02, respectively. As illustrated, the section of the resistor connected between terminals I20 and H3 is disposed to be normally shunted by the engagement of armature contact member 500 and fixed contact member I0! when the resistance bridge is balanced, whereas the section of resistor 94 between terminals 522 and H8 is disposed to be normally connected in circuit with the winding M when the resistance bridge is in a normal balanced condition. The windowpane I0 is similar to the windowpanes E0 of Figs. 1 and 2 except that in this instance, a plurality of wire resistors I24 is utilized being connected through terminal leads and being disposed to be connected in parallel circuit relation with each other. The wire resistors I24 can be embedded in the plastic layer in any of the well-known manners.

Assuming that the circuit is balanced as shown and that the temperature of the windowpane i0 is at the predetermined operating temperature which is to be maintained, then if the temperature of the windowpane should decrease so as to effect a decrease in the resistance of resistor 82, the mid-terminal 02 becomes more positive with respect to the potential of the mid-terminal 00 and current flows from terminal 82 through the winding of relay S0, rectifier 08 to the midterminal 00 of the resistance bridge circuit to so energize the relay 0 3 as to actuate its contact member I00 away from the fixed contact member II'II to connect the section of resistor 9 between terminals I20 and I I8 in circuit with the winding 44 of electromagnet 42. As the magnetic pull of electromagnet 02 is thus decreased, the spring member 40 functions to apply more pressure to the pile 36 of the rheostat 30 to thereby decrease its resistance and increase the how of current in the field winding 22. As the energization of the field Winding 22 is thus increased, the output of the generator It is increased to supply more current to the resistor elements I24 within the windowpane I0 to increase the temperature of the window towards the predetermined. temperature which is to be maintained.

On the other hand, if the temperature within the windowpane It increases from the predetermined temperature which is to be maintained so that the resistance of resistor 82 increases, then the potential at mid-terminal 95 becomes more positive with respect to the potential at mid-terminal 92 and current flows from the terminal 9!] through the rectifier IIl4, the winding of relay N12 to the mid-terminal 92 whereby the relay I02 is energized to efiect a movement of the armature contact member I05 to the right to engage fixed contact member I08 and thereby effectively shunt the section of resistor 95 between terminals II8 and I22 from the winding circuit of the electromagnet 42. As the resistance is thus regulated for the winding circuit of electromagnet 42, the magnetic pull increases to release the pressure on the pile 36 of the rheostat 3G and thereby increase the resistance in circuit with the field winding 22 to decrease the energization thereof. As the energization of the generator I8 is thus decreased, the current flow to the intermediate layer I6 and the resistor wires I24 therein is decreased whereby the temperature developed within the pane Ill is decreased towards the predetermined temperature which is to be maintained.

The system illustrated in Fig. 4 is somewhat similar to the system of Fig. 2, in that a relay II2 is disposed to be energized under predetermined conditions to connect the field winding 22 of the generator. I8 to be energized. Also, a resistor 98 is utilized in circuit relation with the energizing winding 44 of the regulator electromagnet for controlling the degree of energization thereof. In this embodiment, a polarized relay 54 is disposed to be energized to actuate its pivoted armature contact member 66 in a predetermined direction, depending upon the polarity of the potential at mid-terminals 90 and 92, to selectively engage fixed contact members I28 and I36. The pivoted armature member 66 is connected through conductorsIZiZ and 8,8 to supply conductor 32.

The fixed contact member I28 is connected through the energizing winding 534 of a control relay I36 and conductor I38 to supply conductor 34; whereas, the fixed contact member I3@ is connected through the energizing winding Mil of a control relay I42 and conductor i38 to supply conductor 34. The contact member 544 of relay I36 is normally disposed, when the winding I34 is deenergized, in a contact-making position to establish an energizing circuit for the relay II2, which circuit extends from conductor 34 through conductor I38, the energizing winding relay II2, contact member I44 of control-relay i36, and conductors I32 and 88 to the supply conductor 32. When the relay II2 is thus energized, its .movable contact member bridges contact members H4 and IIB to establish the energizing circuit for the field winding 22 of the generator I8.

The contact member I46 of control relay I42 is also normally disposed, when the energizing winding I453 thereof is deenergized, in a contacmaking position to establish a shunt about a section of the resistor 94 to effect an increase in the flow of current through the energizing winding 44 of the regulator electromagnet 42.

In operation, assuming that the resistance bridge including the element 82 is balanced for a given temperature which is to be maintained in thewindowpane Ill, the pivoted armature contact member 66 of the polarized relay E i is in the position shown, and the relay I I2 is therefore energized to close the supply circuit to the generator field winding 22. If the temperature of the windowpane Ill should then decrease from the predetermined temperature which is to be maintained, the resistance bridge becomes unbalanced as described hereinbefore; and, with the windings of the polarized relay 54 disposed thereon in a predetermined manner, as the polarity of the mid-terminal 92 becomes more positive with respect to the polarity of the mid-terminal 9i), current fiows from the terminal 92 through the winding of the relay 54 to the terminal 90 to actuate the armature contact member 66 into engagement with the fixed contact member I30.

When the contact members 66 and I313 are in engagement, the energizing circuit for winding I40 of the control relay I42 is establishedand the relay I42 is so energized that its contact member I46 is actuated to an open circuit position to thereby remove the shunt from the section of the resistor 94 and connect all of the resistor 94 in circuit relation with the energizing winding 44 of the electromagnet 42. As the magnetic pull of the electromagnet 42 is thus decreased, the spring member 4t functions to effect a compression of the stack of discs 36 to decrease the resistance in circuit with the field winding 22. As the field excitation is thus increased, the fiow of current from generator I 8 to the conductive layer I6 is increased to thereby efiect an increase in the temperature of the windowpane III and return the temperature to the predetermined tmperature which is to be maintained.

If the change in the temperature of the windowpane I u should be an'increase, then the polarized relay E i is energized to actuate the armature contact member 65 into engagement with the fixed contact member I23 to establish the energizing circuit for the winding I34 of the control relay I35. When thus energized, the movable contact member I 44 of relay I36 is actuated to a circuit-interrupting position to thereby effect the deenergization of the winding II!) of relay II2. As the relay II2 is thus deenergized, the

contact member thereof drops to a circuit-interrupting position, and the field winding 22 of generator I8 is deenergized. Since the fiow of current from generator I8 then ceases, the temperature of the windowpane Ill decreases to'the predetermind temperature which is to be maintained. Of course, as the resistance bridge including the resistor 82 again becomes balanced by the efiect of the return of the temperature of the windowpane I0 to the predetermined temperature which is to be maintained, the polarized relay 54 becomes deenergized and its pivoted armature contact member 66 is returned to the neutral position shown, and relay I I2 is again energized to establish the supplycircuit for the field winding 22 of the generator I8.

In the systems described hereinbefore, if it is desired to use a temperature sensing element 82 which has a negative temperature coefiicient, such as carbon, the connections of the polarized relays or of the relays polarized by means of directional rectifiers and circuits therewith are reversed from those shown, so as to obtain similar results or operation of the systems to effect the operations as described hereinbefore.

All of the systems described hereinbefore are very sensitive and give a quick response to changes in temperature away from the predetermined temperature which is to be maintained to thereby effect a required defrosting of a windowpane or to in the appended claims, should therefore be given its broadest meaning as covering any heating element or medium for generating heat within the 'windowpane it.

The systems utilize standard equipment and can be readily duplicated. As can be appreciated, it is only necessary that the generator provide a minimum amount of power necessary for maintaining the windows at the desired or required temperature, with the result that there is little power drain from the direct-current system utilized for the field excitation of the generator and an increase in the life of the generator is obtained. The temperature of the window can be maintained at any required temperature, it being found in practice that the resistance bridge can be readily adjusted so as to maintain any predetermined temperature between 70 F. and 120 F. with very slight adjustments of the resistors i6 and 80. As will be readily understood, the system is self-balancing for any given condition of outside temperature.

I claim as my invention:

1. In a system for regulating the temperature of a window having a resistor heating element associated therewith, in combination, a generator connected to be operated to deliver current to the resistor heating element, a field winding for the generator, a rheostat connected in circuit relation with the field winding and disposed to be operated to control the excitation of the generator, a relay having a winding disposed to be energized in response to the operation of the generator to control the operation of the rheostat, a resistor connected in circuit relation with the relay winding, a pair of relays polarized by rectifiers disposed to be selectively energized to control the connections of the resistor in circuit with the relay winding, and a resistance bridge circuit having a resistor disposed in juxtaposition to the window, said juxtaposed resistor having a high temperature coefficient of resistance whereby the resistance varies as the temperature of the window varies, the polarized relays being connected across the resistance bridge circuit to be selectively energized in accordance with the direction of unbalance thereof.

2. In a system for regulating the temperature of a window having a resistor heating element associated therewith, in combination, a generator connected to be operated to deliver current to the resistor heating element, a field winding for the generator, a rheostat connected in circuit relation with the field winding and disposed for operation to control the excitation of the generato-r, a winding having resistance in circuit relation therewi h disposed to be energized in accordance with the operation of the generator to control the operation of the rheostat, means responsive to directional changes in temperature of the window to produce a directional electrical output, means responsive to the directional electrical output occasioned by a decrease in the temperature from a predetermined value for varying the resistance in circuit with the winding, and means responsive to a predetermined electrical output occasioned by an increase in the temperature from a predetermined value for interrupting the field winding circuit of the generator.

8. In a system for regulating the temperature of a window having a resistor heating element associated therewith, in combination, a generator conn cted to "be operated to deliver current-to the resistor heating element, a field winding for the generator, a rheostat connected in circuit relation with the field winding and disposed to be operated to control the excitation of the generator, a relay having a winding disposed to be energized in re- :sponse to the operation of the generator to-con trol the operation of the rheostat,.-a resistor com nected in circuit relation with the relay winding, a resistance bridge circuit having a resistor disposed in juxtaposition to the window, said juxtaposed resistor having a high temperature coefficient of resistance whereby the resistance varies as the temperature of the window varies to give a directional output from the bridge circuit as the temperature of the i indow departs from a predetermined value, a polarized relay disposed to be energized from the bridge circuit as the temperature of the window is decreased below the predetermined value to connect at least a part of the relay resistor in circuit with the relay winding, and another polarized relay disposed to be energized from the bridge circuit as the temperature of the window increases above the prevalue to effect an interruption of the winding circuit of the generator. .n a system for regulating the oi window having a resistor heating element associated therewith, in combination, a generator connected to supply current to the resistor heating element, a field winding for the generator, a rheostat connected in the field winding circuit and disposed for operation to control the excitation of the generator, a contactor disposed when energized to establish the field winding circuit, a winding disposed to be energized to control the operation of the rheostat, a resistor connected in circuit relation with the winding, a pair of relays disposed to be selectively energized, one of the relays being disposed to normally connect said contactor to be energized, the other of the relays being disposed to normally shunt at least a portion of said resistor, means responsive to directional changes in temperature of the window from a predetermined temperature to produce a directional electrical output, and means responsive to the directional electrical. output to selectively energize one or the other of the pair of relays depending upon the direction of departure of the temperature of the window from the predetermined temperature 5. In a system for regulating the temperature of a window having a resistor heating element associated therewith, in combination, a generator connected to supply current to the resistor heating element, a field winding for the generator, a rheostat connected in the field winding circuit and disposed for operation to control the excitation of the generator, a contactor disposed when energized to establish the field winding circuit, a winding disposed to be energized to control the op ration of the rheostat, a resister connected in circuit relation with the winding, a pair of relays disposed to be selectively energized, one of the relays being disposed to normally connect said contactor to be energized, the other of the relays being disposed to normally shunt at least a portion of said resistor, a resistance bridge circuit disposed to be responsive to directional changes in temperature of the window from a predetermined temperature to produce a directional electrical output, and a polarized re- 11 12 lay disposed for operation in response to said directional electrical output to effect the selec- UNITED STATES PATEMS r tive energization of one or the other of the pair N be Name Date of relays depending upon the direction of de- ,121 StOrer May 21, 1907 parture of the temperature of the window from 5 5, 0 Gano June 4, 1929 the predetermined temperature. 9 Melton Aug. 24, 1937 CLARENCE MERSHQN 2,170,193 Godsey, Jr Aug. 22, 1939 2,294,775 Edwards et a] Sept. 1, 1942 REFERENCES CITED 2,311,118 Matthews et a1 Feb. 16, 1943 10 2,399,266 Stiefel Apr. 30, 1946 The followmg references are of record in the 2,427,729 Jenkins Sept. 23 1947 file of this patent: 

